1. Field of the Invention
The present invention generally relates to a power transmission mechanism of a four-wheel drive vehicle, in which a power unit, including an engine and a transmission, is mounted between front wheels and rear wheels, in which a front end part of a power takeoff shaft of the transmission is connected to an input shaft of a differential gear for the front wheels through a propeller shaft, and in which a rear end part of the power takeoff shaft is connected to an input shaft of a final reduction gear for the rear wheels through a propeller shaft, and particularly relates to the power transmission mechanism which is suitable for the four-wheel drive vehicle having a four-wheel independent suspension system.
2. Description of the Related Art
As a power transmission mechanism employed for the aforementioned type of the four-wheel drive vehicle, the mechanism which is disclosed in Japanese Laid-Open Patent Publication No. 2001-301477, has been known. That is, as shown in FIG. 6, each of the right-and-left front wheels 101 and right-and-left rear wheels 102, is suspended independently, by a suspension arm 103 which can swing up and down, and by a shock absorber (not shown). A power unit P, having an engine and a transmission, is mounted between the front wheel 101 and the rear wheel 102.
Each of the differential gear 106 for the front wheels and final reduction gear 107 for the rear wheels, is mounted centrally with respect to a width of a body of the vehicle, and a power takeoff shaft 110 of the transmission is offset to be on the left side of a center line C of the width of the body of the vehicle with respect to a direction in which the vehicle runs forward, and extends in parallel with the centerline C. The front end part of the power takeoff shaft 110 is connected to an input shaft 114 of the differential gear 106 for the front wheels 101 through a propeller shaft 112, which is arranged generally in parallel with the centerline C. On the other hand, the rear end part of the power takeoff shaft 110 is connected to an input shaft 118 of the final reduction gear 107 for the rear wheels 103 through a propeller shaft 115, which tilts with respect to the centerline C.
The power unit P is mounted slightly rearwards relative to a center between the front wheel 101 and the rear wheel 102. In the arrangement, the distance between the final reduction gear 107 and the rear end part of the power takeoff shaft 110, is shorter than the distance between the differential gear 106 and the front end part of the power takeoff shaft 110.
The input shaft 118 of the final reduction gear 107, is arranged in parallel with the centerline C. The front end part of the propeller shaft 115 is coupled to the rear end part of the power takeoff shaft 110 through a universal joint 120 with a predetermined tilt angle therebetween. On the other hand, the rear end part of the propeller shaft 115 is coupled to the input shaft 118 of the final reduction gear 107 through a universal joint 121 with a predetermined tilt angle therebetween, too.
In the arrangement, the two universal joints 120 and 121 locating at the front and rear end parts of the propeller shaft 115, are non-constant velocity universal joints, respectively. However, since both of the universal joints 120 and 121 are constructed symmetrical to each other, the changes in their rotational speeds during one rotation, are cancelled to each other. Therefore, the rotation is transmitted between the drive takeoff shaft 110 and the input shaft 118, generally at an equal speed.
In the four-wheel drive vehicle having a four-wheel independent suspension, each of the differential gear 106 and the final reduction gear 107, is mounted at a center with respect to the width of the vehicle, as aforementioned. With the arrangement, it is possible to make length of each of the suspension arms (A-shaped arms) 103 equal in a direction of right and left, and also possible to make each thereof longer. Namely, with the arrangement, the width of the vehicle is prevented from growing in size, and a suspension stroke thereof is secured to be long while suppressing any change in tread so that comfortable ride of the vehicle is realized.
However, in the power transmission mechanism, the drive shaft 110 and the input shaft 118 are connected by the propeller shaft 115, tilted, via the two universal joints 120, 121, as explained above. That is, according to the arrangement, not only crossed axes angle between the propeller shaft 115 and the drive shaft 110 which are connected to each other via one 120 of the two universal joints and between the propeller shaft 115 and the input shaft 118 which are connected to each other via the other 121 of the two universal joints, become greater, but also the rotational speed of the power transmission mechanism is changed twice through the pair of universal joints 120 and 121. Therefore, with the arrangement, it is difficult to reduce the loss of driving power which is transmitted to the rear wheels 102. Also, the arrangement necessitates a counter-measure against short life of the universal joints 120 and 121.
Also, according to the arrangement, the power unit P is mounted a bit backward (or rearward) with respect to a center between the front wheel 101 and the rear wheel 102. Therefore, the distance between the power takeoff shaft 110 and the input shaft 118, in which the propeller shaft 115 is mounted, becomes inevitably smaller. Therefore, the aforementioned crossed axes angle of the propeller shaft becomes larger.
Therefore, it is an object of the present invention to provide a power transmission mechanism of a four-wheel drive vehicle, in which a loss of a driving power that is transmitted to rear wheels of the vehicle is reduced and a joint part (or coupling part) in the power transmission mechanism is simplified in construction while maintaining a good ride feeling and a compactness of the vehicle.
In accomplishing the above and other objects, there is provided a power transmission mechanism of a four-wheel drive vehicle, comprising: a power unit which has an engine and a transmission, in which the power unit is provided between front wheels and rear wheels; a power takeoff shaft of the transmission, in which the power takeoff shaft is provided generally in parallel and in offset with a centerline relative to a width of a body of the vehicle; a differential gear for the front wheels, in which the differential gear is provided centrally relative to the width of the body, and in which the differential gear has an input shaft provided in parallel with the centerline and provided coaxially with the power takeoff shaft; a front propeller shaft for connecting between a front part of the power takeoff shaft and a rear part of the input shaft of the differential gear; a final reduction gear for the rear wheels, in which the final reduction gear is provided centrally relative to the width of the body of the vehicle, in which the final reduction gear has an input shaft provided such that a front part of the input shaft tilts toward the power takeoff shaft with respect to the centerline; and a rear propeller shaft for connecting between a rear part of the power takeoff shaft and the front part of the input shaft of the final reduction gear, wherein the front propeller shaft is arranged so as to be coaxial with the power takeoff shaft and the input shaft of the differential gear, and wherein the rear propeller shaft is arranged so as to be coaxial with the input shaft of the final reduction gear.
According to the mechanism, since the front part of the input shaft of the final reduction gear tilts toward the power takeoff shaft with respect to the centerline of the vehicle, and since the propeller shaft for the rear wheels is generally coaxial with the input shaft of the final reduction gear, not only the downsizing, weight saving and simplification of the joint part (or coupling part) can be realized, but also the loss of driving power that is transmitted to the rear wheels is reduced while the abrasion of the joint part is effectively prevented. In addition, the right and left suspension arms for the front and rear wheels can be set not only to be equal to each other, but also to be longer, respectively. Thereby, the comfortable ride and the compactness in the direction of width of the body of the vehicle are maintained.
In the above mechanism, preferably, the power unit is provided between the front wheels and the rear wheels such that a total length of the rear propeller shaft and the input shaft for the final reduction gear is longer than a total length of the front propeller shaft and the input shaft for the differential gear.
According to the mechanism, the angle of inclination, or tilt, of the rear propeller shaft becomes smaller. Therefore, with the mechanism, the loss of driving power that is transmitted to the rear wheels is effectively reduced, and the assemblage thereof is facilitated.
In the mechanism, preferably, there is further provided a braking device which is mounted on the input shaft of the final reduction gear.
With the mechanism, the space in front of the final reduction gear for the rear wheels, can be made full use of for mounting the braking device. Especially, in a case that the power unit is provided between the front wheels and the rear wheels such that the distance between the power unit and the final reduction gear is secured to be relatively greater, it is possible to secure enough space for mounting the brake device. Also, with the mechanism that the braking device is positioned in front of the final reduction gear, the effect to cool down the braking device is enhanced, by getting the air coming from the fore of the vehicle while it is running.
In the mechanism, preferably, the braking device is a wet multiple-disk braking device.
With the mechanism, it is possible to make compact the dimension of the braking device in the direction of radius, especially the dimension in the direction of up and down thereof. Consequently, with the mechanism, enough space is secured between the braking device and the ground, and the braking device is maintained at a high location, as a minimum height, with respect to the ground level.
In the mechanism, preferably, the wet multiple-disk braking device has a casing which is integrated with a casing which the final reduction gear has.
With the mechanism, the casing of the braking device can be simplified. Also, with the mechanism, the same lubricant (or engine oil) can be employed, commonly, as the lubricant circulating in the final reduction gear and as the lubricant circulating in the braking device. Therefore, the maintenance of the lubrication system in the power transmission mechanism can be minimized.